We have developed an over-the-horizon radar (OTHR ) clutter prediction model to better define equatorial clutter environments. This physics-based model is referred to as the clutter effects model, or CLEM. It is nominally configured toward simulation of the Navy's Relocatable OTHR (ROTHR). Radar signal propagation geometry is computed by ray tracing through a global ionosphere model (driven by geophysical conditions, time, and location). The equatorial ionosphere has high levels of field-aligned irregularities which produce strong clutter environments for OTHR. Strengths of modeled ionospheric irregularities are derived from transionospheric scintillation measurement data. Clutter Doppler and delay spreading is analyzed using multiple phase screen forward diffraction techniques. Technical background on an earlier version of CLEM, which performed three-dimensional (3-D) ray tracing, is given by Lauer et al. . The current version, which is under discussion here, has been sped up through the use of 2-D ray tracing and substitution of a simpler spectral form for the signal delay-Doppler spectrum; however, the reader may refer to Lauer et al  for background on various aspects of the theory which still pertain, including phase diffraction propagation effects, absorption calculations, and ionospheric structure predictions. This paper describes CLEM models and compares CLEM predictions with clutter measurement data for specific cases.
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